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Question

An infinite row of boxes is arranged. Each box has half the volume of the previous box. If the largest box has a volume of $20\text{ cc}$, what is the total volume of all the boxes'?

The correct answer is
$40\text{ cc}$

Infinite Series Volume Calculation

This problem involves finding the sum of an infinite geometric series. The volumes of the boxes form a sequence where each term is half of the previous one.

Identifying Series Parameters

  • The first term (largest box volume), a = $20\text{ cc}$.
  • The common ratio, r, is the factor by which each volume decreases. Since each box has half the volume of the previous one, r = $1/2$.

Sum of Infinite Geometric Series

The formula for the sum (S) of an infinite geometric series is:

$ S = \frac{a}{1-r} $

This formula is valid only if the absolute value of the common ratio is less than 1 (i.e., $|r| < 1$). In this case, $|1/2| < 1$, so the formula can be applied.

Calculating Total Volume

Substitute the values of a and r into the formula:

$ S = \frac{20}{1 - \frac{1}{2}} $

First, calculate the denominator:

$ 1 - \frac{1}{2} = \frac{1}{2} $

Now, perform the division:

$ S = \frac{20}{\frac{1}{2}} = 20 \times 2 = 40 $

The total volume of all the boxes is $40\text{ cc}$.

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Important Questions from Progression (Notes)

  1. The sum of 16 terms of the series $\sqrt{2} + \sqrt{8} + \sqrt{18} + \sqrt{32} + .....$ is :
  2. The $5^{\text{th}}$ and $9^{\text{th}}$ terms of an arithmetic progression are 7 and 13 respectively. What is the $15^{\text{th}}$ term?
  3. Find the sum of the G.P.:
    $5/11, 5/121, 5/1331, 5/14641, ...$ to $n$ terms.
  4. A ball is dropped from a height of 100 m. The ball after each bounce rises vertically by half its previous height (This means at the first bounce it rises by 50 m, by 25 m at the second bounce and so on). What is the vertical distance travelled by the ball between the first and the fifth bounces?
  5. An ant starts at the origin and moves along the $y$-axis and covers a distance $l$. This is its first stage in its journey. Every subsequent stage requires the ant to turn right and move a distance which is half of its previous stage. What would be its coordinates at the end of its $5^{\text{th}}$ stage?
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